The present invention relates to a circuit for detecting a particular frequency out of a signal coming in over a telephone line or similar communication path and, more particularly, to a frequency detection circuit applicable to a facsimile terminal or similar data terminal using a telephone line.
It has been customary to detect a particular frequency component out of a signal received over, for example, a telephone line by using a band-pass filter, frequency detector, and controller. The band-pass filter has a pass band equivalent to the standard of the band width of a desired frequency. A signal from an input terminal is applied to the band-pass filter. The filter selectively delivers the input signal to the frequency detector. The frequency detector determines the power level of the input signal and, if it is higher than a predetermined level, sends a detection signal to the controller. In response, the controller determines that the desired frequency has been detected. However, such a frequency detection scheme has the following problem. Assume that the input signal coming in through the input terminal contains switching noise attributable to an exchange and extraneous noise introduced into a telephone line, e.g., the power level of noise contains substantially constant white noise without regard to frequency. Then, if the power level of the input signal is detected by the frequency detector via the filter, the controller is apt to determine that the desired frequency has been detected, despite that the frequency should not be detected.
Another conventional approach for frequency detection is to connect a plurality of band-pass filters each having a particular pass band to an input terminal in parallel, as taught in Japanese Patent Laid-Open Publication Nos. 59-2503 and 63-155848 by way of example. Although this kind of approach promotes effective frequency detection, it also involves detection errors attributable to white noise.